Microelectronic devices are generally fabricated on semiconductor substrates as integrated circuits. A complementary metal-oxide-semiconductor (CMOS) field effect transistor is one of the core elements of the integrated circuits. Dimensions and operating voltages of CMOS transistors are continuously reduced, or scaled down, to obtain ever-higher performance and packaging density of the integrated circuits.
One of the problems due to the scaling down of CMOS transistors is that the power consumption keeps increasing. This is partly because leakage currents are increasing (e.g. due to short-channel effects) and because it becomes difficult to decrease the supply voltage. The latter is mainly due to the fact that the subthreshold swing is limited to minimally about 60 mV/decade, such that switching the transistor from ON to OFF needs a certain voltage variation and therefore a minimum supply voltage.
A potential candidate to replace the MOSFET in future technology nodes is a Tunnel Field Effect Transistor (TFET) because a TFET does not have a limit to its subthreshold swing, and therefore it holds the promise to operate at a supply voltage well below 1 V. However, not all TFETs beat the 60 mV/dec limit. The smaller the bandgap of especially the TFET source material and the thinner the gate dielectric, the more likely the device will beat the 60 mV/dec limit [A. S. Verhulst et al., “Boosting the on-current of a n-channel nanowire tunnel field-effect transistor by source material optimization”, J. Appl. Phys. 104, 064514 (2008)]. An all-silicon TFET e.g. has an average subthreshold swing of about 200 mV/dec.
Suggestions have been made in literature to improve the subthreshold swing of the TFET such as changing the material of the TFET to a smaller bandgap material, however this is a technological challenge because it is necessary to further develop the technology, find compatibility with the widely-used Si technology and find a good gate-dielectric material if this material forms part of the channel.
Another solution suggested in literature is a further decrease in gate-dielectric thickness which is also a technological challenge which probably requires exploring further higher-k gate dielectric materials in order to ensure that the gate leakage currents remain low.
As a conclusion there still exists a problem to improve the subthreshold swing of a TFET without having to change the TFET material and without having to further decrease the gate-dielectric thickness.